[Haskell-cafe] why does "iterateNTimes (10^6) (ap . liftM (:) . return $ 0) (return [])" produce output in the IO monad, but stack overflow in the maybe monad?

Can someone explain why the one stack overflows and the other one doesn't? iterateNTimes i f x = foldr (.) id (replicate i f) $ x tntIO :: IO Int -- same as replicateM (10^6) $ return 0 , and same as sequence . replicate (10^6) $ return 0 tntIO = iterateNTimes (10^6) (ap . liftM (:) . return $ ) (return []) -- produces output tntMb :: Maybe Int -- overflows tntMb = iterateNTimes (10^6) (ap . liftM (:) . return $ ) (return []) -- stack overflow